Why is it important that devices in a high availability pair share the same configuration?

Spanning tree loop prevention is handled by STP protocol itself and is not a reason HA pairs require matching configurations.

When a failover occurs, the standby device immediately becomes the active device. If configurations differ, the standby device would lack the necessary policies, routes, NAT rules, or session state to properly handle existing connections, causing application traffic to drop rather than resume gracefully.

Configuration synchronization in HA is not instant between devices in the sense described - the purpose is failover readiness, not real-time config propagation speed.

Load sharing (active-active) is one HA mode but is not the primary reason configurations must match; even active-passive HA requires matching configs for failover correctness.